Various algorithms have been developed for estimating channel quality at a wireless receiver. These algorithms are required, for example, in systems using the Third Generation Partnership Project (3GPP) High Chip Rate Time Division Duplex (TDD) mode, the 3GPP Low Chip Rate TDD mode, the 3GPP Frequency Division Duplex (FDD) mode, the time division—synchronous code division multiple access (TD-SCDMA) standard, and High Speed Downlink Packet Access (HSDPA) extensions of the aforementioned systems. The quality estimates may be used for transmit power control, in- and out-of-synchronization decisions, radio link failure decisions, and channel quality indicators (CQIs) to support adaptive modulation and coding (AMC) techniques.
In many wireless communication systems, transport channels are grouped, interleaved, coded and mapped onto groups of physical channels to form a composite channel. In the 3GPP standards, the composite channel is called a Coded Composite Transport Channel (CCTrCH).
A block of received data in a composite channel may be mapped onto a plurality of codes in a plurality of timeslots (also called slots) and/or a plurality of frames. A wireless receiver may use multi-user detection (MUD), a Rake receiver, or any type of demodulator to compute soft symbol values associated with the composite channel. Each partial data block, mapped onto one code, in one slot, in one frame, is referred to as a data burst. Multiple bursts can be transmitted and received simultaneously in code division multiple access (CDMA) systems. Symbols from each data burst are provided separately by the demodulator. Typically, a complete block of received data is spread over multiple bursts. Statistics are computed for each of these bursts, and then combined to provide a composite channel quality estimate. In cases where channel quality estimates are required and no data bursts are available or otherwise indicated, the same techniques can be applied to bursts carrying broadcast or pilot signals.
In order to implement power control in the 3GPP TDD standards, uplink (UL) and downlink (DL) CCTrCHs are paired. The quality of the DL CCTrCHs is monitored by the User Equipment (UE), whereas the Node B monitors the UL CCTrCHs. Various parameters, including the quality of the CCTrCHs, must be monitored to determine if an UL-DL group of CCTrCHs between a UE and Node B is of sufficiently high quality to remain in use. If the quality of the group of CCTrCHs is sufficiently high, the CCTrCHs are declared ‘In-Sync’; whereas if the quality is insufficient, the CCTrCHs are declared ‘Out-Of-Sync’. The ‘Out-Of-Sync’ condition eventually leads to a declaration of a radio link failure, whereby radio resources supporting the failed link are released.
A shortcoming of prior art channel estimation techniques is that they do not provide sufficiently accurate estimates of channel quality at the receiver. Inaccurate channel quality estimation can cause erroneous such as ‘In-Sync’ and ‘Out-Of-Sync’ declarations, reduce throughput, waste transmit power, and increase interference to other cells.
For 3GPP systems employing HSDPA, the high speed downlink shared channel (HS-DSCH) is a composite channel on which quality is computed to form a CQI as defined by 3GPP. The quality of the HS-DSCH channel is monitored at the UE, a CQI is computed, then it is reported back to the Node B. The CQI effectively provides a recommended coding and modulation scheme for the AMC function. If the CQI is not accurate, the recommended coding and modulation scheme will be suboptimal. Overestimating channel quality can cause the UE and Node B to continue attempting to use a modulation and coding scheme when reception quality is too poor to justify their continued use. Underestimation of channel quality may lead to excessive transmit power or ultimately a premature declaration of radio link failure and release of radio resources. Thus, a call may be dropped without cause. Excessive transmit power will, in turn, lead to a system-level throughput loss, due to the fact that interference in other cells may increase needlessly.
Accordingly, what is needed is an improved method for estimating channel quality which is devoid of the disadvantages of prior art techniques.